Fix build breakage caused by the recent toString() commit.

tests/corelib/src/CollectionToStringTest.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Tests for the toString methods on collections (including maps).
  */
 
 // TODO(jjb): seed random number generator when API allows it
 
@@ skipping 236 matching lines @@
 }
 
 /**
  * Generates a random element which can be an int, a collection, or a map,
  * and emits it to StringRep. The beingMade parameter is a list of collections
  * currently under construction, i.e., candidates for recursive references.
  *
  * If exact is true, the returned element will not be, and will not contain
  * a collection with ill-defined iteration order (i.e., a HashSet or HashMap).
  */
-void randomElement(int size, bool exact, StringBuffer stringRep,
+Object randomElement(int size, bool exact, StringBuffer stringRep,

[jjb, 2012/02/22 04:55:06]

I find it astonishing that the compiler never told me about this!  I did a lot
testing, with type-checking enabled.

     List beingMade) {
   Object result;
   double elementTypeFrac = Math.random();
   if (elementTypeFrac < 1/3) {
     result = random(1000);
     stringRep.add(result);
   } else if (elementTypeFrac < 2/3) {
     // Element Is a random (new) collection
     result = randomCollectionHelper(size, exact, stringRep, beingMade);
   } else {
@@ skipping 16 matching lines @@
 bool randomBool() {
   return Math.random() < .5;
 }
 
 /** Returns the alphabetized characters in a string. */
 String alphagram(String s) {
   List<int> chars = s.charCodes();
   chars.sort((int a, int b) => a - b);
   return new String.fromCharCodes(chars);
 }